Carbon Dioxide Gas Phase Reduction Apparatus and Method

ABSTRACT

A carbon dioxide gas phase reduction device includes an oxidation tank including an oxidation electrode, a reduction tank to which carbon dioxide is supplied, an intermediate tank that is disposed between the oxidation tank and the reduction tank and capable of pouring and discharging an electrolytic solution, an ion exchange membrane disposed between the oxidation tank and the intermediate tank, a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank, and a conducting wire connecting the oxidation electrode to the reduction electrode.

TECHNICAL FIELD

The present invention relates to a carbon dioxide gas phase reduction device and a carbon dioxide gas phase reduction method.

BACKGROUND ART

Conventionally, a carbon dioxide gas phase reduction device has been researched and developed. A carbon dioxide gas phase reduction device disclosed in NPL 1 is configured by disposing a gas reduction sheet, in which an ion exchange membrane (Nafion (registered trademark)) and a reduction electrode (Cu) are combined, between a left oxidation tank and a right reduction tank. The ion exchange membrane is disposed toward the oxidation tank side, and the reduction electrode is disposed toward the reduction tank side. The oxidation tank is filled with a potassium hydroxide (KOH) aqueous solution as an electrolytic solution, and an aluminum gallium nitride (AlGaN) oxidation electrode in which a nickel oxide (NiO) catalyst is laminated is immersed in the aqueous solution. The oxidation electrode is connected to the reduction electrode of the gas reduction sheet via a conducting wire.

In such a carbon dioxide gas phase reduction device, when helium (He) is put into the potassium hydroxide aqueous solution in the oxidation tank and carbon dioxide (CO₂) is put into the reduction tank, and the oxidation electrode is irradiated with light (hv), a carbon dioxide reduction reaction proceeds at a three-phase interface consisting of the ion exchange membrane (Nafion) of the gas reduction sheet, the reduction electrode (Cu), and the carbon dioxide (CO₂) in the reduction tank. Specifically, in the oxidation tank, oxygen is generated due to an oxidation reaction of water. In the reduction tank, hydrogen is generated by a reduction reaction of protons in the ion exchange membrane, and carbon monoxide and formic acid are generated by a carbon dioxide reduction reaction.

CITATION LIST Non Patent Literature

-   [NPL 1] Sato, et al. (3 persons), “Photoelectrochemical properties     of CO₂ reduction reaction using proton exchange membrane with Cu     electrode”, Electrochemical Autumn Meeting, Oral presentation, 2019,     1B05.

SUMMARY OF THE INVENTION Technical Problem

However, since the potassium hydroxide aqueous solution, which is the electrolytic solution, is always in contact with the reduction electrode via the ion exchange membrane, the reduction electrode deteriorates, a reaction field of the carbon dioxide reduction reaction is lost, and a lifespan of the carbon dioxide reduction reaction is reduced. In this regard, a method in which the potassium hydroxide aqueous solution is poured into the oxidation tank when an operation of the gas phase reduction device is started, and the potassium hydroxide aqueous solution is discharged from the oxidation tank when the operation is stopped, and thus contact between the potassium hydroxide aqueous solution and the reduction electrode is limited to only during an operation of the gas phase reduction device is also conceivable. However, in the case of this method, oxygen generated in the oxidation tank is released from the oxidation tank when a valve is opened and closed, which makes recovery difficult.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique capable of inhibiting deterioration of a reduction electrode and improving a lifespan of a carbon dioxide reduction reaction on the reduction electrode.

Means for Solving the Problem

A carbon dioxide gas phase reduction device according to one aspect of the present invention includes: an oxidation tank including an oxidation electrode; a reduction tank to which carbon dioxide is supplied; an intermediate tank that is disposed between the oxidation tank and the reduction tank and is capable of pouring and discharging an electrolytic solution; an ion exchange membrane disposed between the oxidation tank and the intermediate tank; a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank; and a conducting wire connecting the oxidation electrode to the reduction electrode.

A carbon dioxide gas phase reduction method according to one aspect of the present invention is a carbon dioxide gas phase reduction method performed by a carbon dioxide gas phase reduction device, the carbon dioxide gas phase reduction device including: an oxidation tank including an oxidation electrode: a reduction tank to which carbon dioxide is supplied; an intermediate tank that is disposed between the oxidation tank and the reduction tank and is capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution; an ion exchange membrane disposed between the oxidation tank and the intermediate tank; a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank; and a conducting wire connecting the oxidation electrode to the reduction electrode, in which the method includes performing a first step of pouring the electrolytic solution into the oxidation tank and supplying carbon dioxide to the reduction tank, and a second step of pouring the electrolytic solution into the intermediate tank only when the oxidation electrode is irradiated with light.

A carbon dioxide gas phase reduction method according to one aspect of the present invention is a carbon dioxide gas phase reduction method performed by a carbon dioxide gas phase reduction device, the carbon dioxide gas phase reduction device including: an oxidation tank including an oxidation electrode; a reduction tank to which carbon dioxide is supplied; an intermediate tank that is disposed between the oxidation tank and the reduction tank and is capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution; an ion exchange membrane disposed between the oxidation tank and the intermediate tank; a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank; and a conducting wire connecting the oxidation electrode to the reduction electrode, in which the method includes performing a first step of pouring the electrolytic solution into the oxidation tank and supplying carbon dioxide to the reduction tank, and a second step of pouring the electrolytic solution into the intermediate tank only when a voltage is applied between the oxidation electrode and the reduction electrode.

Effects of the Invention

According to the present invention, it is possible to provide a technique capable of inhibiting deterioration of a reduction electrode and improving a lifespan of a carbon dioxide reduction reaction on the reduction electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram showing a configuration of a carbon dioxide gas phase reduction device according to Examples 1 to 4.

FIG. 2 is a diagram showing a method for producing a gas reduction sheet using an electroless plating method.

FIG. 3 is a diagram showing a light irradiation time and a voltage application time used in Examples 1 and 5.

FIG. 4 is a diagram showing a light irradiation time and a voltage application time used in Examples 2 and 6.

FIG. 5 is a diagram showing a light irradiation time and a voltage application time used in Examples 3 and 7.

FIG. 6 is a diagram showing a light irradiation time and a voltage application time used in Examples 4 and 8.

FIG. 7 is a configuration diagram showing a configuration of a carbon dioxide gas phase reduction device according to Examples 5 to 8.

FIG. 8 is a configuration diagram showing a configuration of a conventional carbon dioxide gas phase reduction device.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the figures. In the description of the figures, the same portions are denoted by the same reference numerals and the description thereof will be omitted.

SUMMARY OF THE INVENTION

The present invention is an invention relating to a carbon dioxide gas phase reduction device that causes a carbon dioxide reduction reaction by irradiating light or causes a carbon dioxide electrolytic reduction reaction to improve a lifespan of the reduction reaction and falls within the technical fields of fuel generation techniques and solar energy conversion techniques.

The present invention uses a gas reduction sheet in which a reduction electrode is directly formed on an ion exchange membrane and directly supplies gas phase carbon dioxide to a surface of the reduction electrode, and thus an intermediate tank capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution is disposed between an oxidation tank and a reduction tank in a carbon dioxide gas phase reduction device that reduces carbon dioxide. Also, the electrolytic solution is poured into the intermediate tank and brought into contact with the reduction electrode to cause the carbon dioxide reduction reaction in the reduction electrode of the gas reduction sheet only when an oxidation electrode in the oxidation tank is irradiated with light or when a voltage is applied between the oxidation electrode and the reduction electrode.

As a result, deterioration of the reduction electrode can be inhibited, and the lifespan of the carbon dioxide reduction reaction on the reduction electrode can be improved. Since the electrolytic solution in the oxidation tank is not released, it does not interfere with the recovery of oxygen generated by the oxidation electrode in the oxidation tank.

Example 1 Carbon Dioxide Gas Phase Reduction Device

FIG. 1 is a configuration diagram showing a configuration of a carbon dioxide gas phase reduction device according to Example 1.

A carbon dioxide gas phase reduction device includes an oxidation tank 2 including an oxidation electrode 1, a reduction tank 3 to which carbon dioxide (CO₂) is supplied, an intermediate tank 4 that is disposed between the oxidation tank 2 and the reduction tank 3 and capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution, an ion exchange membrane 5 disposed between the oxidation tank 2 and the intermediate tank 4, a gas reduction sheet 100 in which an ion exchange membrane 6 and a reduction electrode 7 are laminated and which is disposed between the reduction tank 3 and the intermediate tank 4 with the ion exchange membrane 6 facing the intermediate tank 4 and the reduction electrode 7 facing the reduction tank 3, a conducting wire 8 connecting the oxidation electrode 1 to the reduction electrode 7, and a light source 9 irradiating the oxidation electrode 1 with light. The details will be described below.

The oxidation electrode 1 is configured by epitaxially growing a thin film of n-type gallium nitride (n-GaN), which is an n-type semiconductor, and aluminum gallium nitride (AlGaN) in the order of n-GaN and AlGaN on a sapphire substrate, and performing vacuum vapor deposition of nickel (Ni) and heat treatment thereon, thereby forming a co-catalyst thin film of nickel oxide (NiO). Thus, the oxidation electrode 1 of NiO/AlGaN/n-GaN/sapphire is formed. NiO is a catalyst layer. AlGaN is a light absorption layer. This oxidation electrode 1 is installed to be immersed in an aqueous solution 10 which is the electrolytic solution poured into the oxidation tank 2. The aqueous solution 10 in the oxidation tank 2 is a 1 mol/L potassium hydroxide (KOH) aqueous solution.

The oxidation tank 2 and the intermediate tank 4 are separated from each other by the ion exchange membrane 5. An aqueous solution 11 serving as the electrolytic solution poured into the intermediate tank 4 is also a 1 mol/L potassium hydroxide aqueous solution. The intermediate tank 4 and the reduction tank 3 are separated from each other by the gas reduction sheet 100 in which the reduction electrode 7 is directly formed on the ion exchange membrane 6. The ion exchange membrane 6 is disposed on the intermediate tank 4 side, and the reduction electrode 7 is disposed on the reduction tank 3 side. Nafion (registered trademark) is used for both the ion exchange membrane 5 disposed between the oxidation tank 2 and the intermediate tank 4 and the ion exchange membrane 6 of the gas reduction sheet 100 disposed between the intermediate tank 4 and the reduction tank 3. Copper (Cu) is used for the reduction electrode 7.

The oxidation electrode 1 immersed in the aqueous solution 10 in the oxidation tank 2 and the reduction electrode 7 of the gas reduction sheet 100 disposed toward the reduction tank 3 are connected to each other by the conducting wire 8.

A 300 W high-pressure xenon lamp is used as the light source 9. Light output from the light source 9, in which wavelengths of 450 nm or more are cut, is used.

FIG. 2 is a diagram showing a reaction system of an electroless plating method used as a method for producing the gas reduction sheet 100. One surface of the ion exchange membrane 6 is polished. Further, in order to improve proton mobility of the ion exchange membrane 6, the ion exchange membrane 6 is immersed in each of boiling nitric acid and boiling pure water. Two left and right tanks 21 and 22 are filled with a plating solution 31 and a reduction agent 32 shown in Table 1, respectively.

TABLE 1 Plating solution Reduction agent CuSo₄•5H₂O 3.5 g/L NaBH₄ 10 vol. % KNaC₄H₄O₆•H₂O 34.0 g/L NaOH 7.0 g/L Na₂CO₃ 3.0 g/L

The tank 21 and the tank 22 are separated from each other by the ion exchange membrane 6. The ion exchange membrane 6 is disposed with the polished surface facing the plating solution 31 side. Since NaBH₄, which is a main component of the reduction agent 32, is a polar compound, it permeates the ion exchange membrane 6. The following oxidation-reduction reaction occurs and copper (Cu) is deposited at an interface between the plating solution 31 and the polished surface of the ion exchange membrane 6, and thus the gas reduction sheet 100 in which the reduction electrode is formed on the ion exchange membrane 6 can be obtained.

BH₄ ⁻+4OH⁻→BO₂ ⁻+2H₂O+2H₂+4e ⁻

Cu²⁺+2e ⁻→Cu

Carbon Dioxide Gas Phase Reduction Method

Next, a carbon dioxide gas phase reduction method performed by the carbon dioxide gas phase reduction device will be described.

First, a NiO-forming surface of the oxidation electrode 1 is fixed toward the light source 9 such that the NiO-forming surface of the oxidation electrode 1 serves as a light receiving surface (a first step). A light receiving area of the oxidation electrode 1 was set to 3.8 cm².

Next, the 1 mol/L potassium hydroxide aqueous solution 10 is poured into the oxidation tank 2 (a second step).

Next, a tube 12 is put into the aqueous solution 10 in the oxidation tank 2 and helium (He) is flowed into the aqueous solution 10 at a flow rate of 5 ml/min, and carbon dioxide (CO₂) is flowed into the reduction tank 3 from a gas input port 13 at the same flow rate (a third step).

Next, after the oxidation tank 2 and the reduction tank 3 are sufficiently replaced with helium and carbon dioxide, respectively, the 1 mol/L potassium hydroxide aqueous solution 11 is poured into the intermediate tank 4 from the aqueous solution input port 14, and the oxidation electrode 1 is uniformly irradiated with the light using the light source 9 (a fourth step).

In this case, the carbon dioxide reduction reaction proceeds at a three-phase interface configured of the ion exchange membrane (Nafion) 6 in the gas reduction sheet 100, the reduction electrode (Cu) 7, and gas phase carbon dioxide (CO₂). A surface area of the reduction electrode 7 to which carbon dioxide is directly supplied is about 6.8 cm².

Finally, after the light irradiation of the oxidation electrode 1 is completed, the aqueous solution 11 is discharged from an aqueous solution output port 15 of the intermediate tank 4 (a fifth step).

The “first step” described in “Claims” corresponds to the second step and the third step. The second step and the third step may be implemented at the same timing. A step of flowing carbon dioxide into the reduction tank 3 may be carried out at a timing prior to the step of pouring the aqueous solution 10 into the oxidation tank 2. Further, the “second step” described in “Claims” corresponds to the fourth step and the fifth step.

In Example 1, the oxidation electrode 1 was irradiated with light having a profile-set illuminance as shown in FIG. 3 . A process of “irradiating the oxidation electrode 1 with light having an illuminance of 2.2 mW/cm² in a wavelength region of 365 nm or more for 1 hour and then waiting for 1 hour without irradiating light” is repeated, and the reaction is caused to proceed until a net light irradiation time reaches 3 hours. Further, when the light irradiation is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the light irradiation is started again, the aqueous solution 11 is poured into the intermediate tank 4.

Concentration analysis of gas products in each reaction tank was performed using a gas chromatograph at an arbitrary time during the light irradiation. In particular, a liquid product in the reduction tank 3 was subjected to the concentration analysis using a liquid chromatograph. As a result, it was confirmed that oxygen was generated in the oxidation tank 2 by the oxidation reaction of water using holes. In the reduction tank 3, it was confirmed that hydrogen was produced by the reduction reaction of protons using electrons, and carbon monoxide, formic acid, formaldehyde, methane, ethylene, methanol, and ethanol were produced by the reduction reaction of carbon dioxide.

Example 2

Example 2 also uses the same carbon dioxide gas phase reduction device as in Example 1 shown in FIG. 1 . In Example 2, the oxidation electrode 1 was irradiated with light having a profile-set illuminance as shown in FIG. 4 . A process of “irradiating the oxidation electrode 1 with light having an illuminance of 2.2 mW/cm² in the wavelength region of 365 nm or more for 1 hour and then waiting for 3 hours without irradiating light” is repeated, and the reaction is caused to proceed until the net light irradiation time reaches 3 hours. Further, when the light irradiation is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the light irradiation is started again, the aqueous solution 11 is poured into the intermediate tank 4. Other points are the same as in Example 1.

Example 3

Example 3 also uses the same carbon dioxide gas phase reduction device as in Example 1 shown in FIG. 1 . In Example 3, the oxidation electrode 1 was irradiated with light having a profile-set illuminance as shown in FIG. 5 . A process of “irradiating the oxidation electrode 1 with light having an illuminance of 2.2 mW/cm² in the wavelength region of 365 nm or more for 1 hour and then waiting for 5 hours without irradiating light” is repeated, and the reaction is caused to proceed until the net light irradiation time reaches 3 hours. Further, when the light irradiation is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the light irradiation is started again, the aqueous solution 11 is poured into the intermediate tank 4. Other points are the same as in Example 1.

Example 4

Example 4 also uses the same carbon dioxide gas phase reduction device as in Example 1 shown in FIG. 1 . In Example 4, the oxidation electrode 1 was irradiated with light having a profile-set illuminance as shown in FIG. 6 . A process of “irradiating the oxidation electrode 1 with light having an illuminance of 2.2 mW/cm² in the wavelength region of 365 nm or more for 1 hour and then waiting for 10 hours without irradiating light” is repeated, and the reaction is caused to proceed until the net light irradiation time reaches 3 hours. Further, when the light irradiation is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the light irradiation is started again, the aqueous solution 11 is poured into the intermediate tank 4. Other points are the same as in Example 1.

Example 5

FIG. 7 is a configuration diagram showing a configuration of a carbon dioxide gas phase reduction device according to Example 5.

In Example 5, a power source 16 is used instead of the light source 9. The power source 16 is inserted on a path of a conducting wire 8. Further, since the oxidation electrode 1 does not need to receive light, the oxidation electrode 1 of Example 5 was configured by using platinum (manufactured by Nilaco Corporation). A surface area of the oxidation electrode 1 was set to about 0.55 cm². Other configurations are the same as in Example 1.

After the oxidation tank 2 and the reduction tank 3 were sufficiently replaced with helium and carbon dioxide, respectively, the power source 16 was connected between the oxidation electrode 1 and the reduction electrode 7 with the conducting wire 8, and a voltage of 1.5 V was applied to pass a current. Other procedures are the same as in Example 1.

That is, in Example 5, a profile-set voltage was applied to the conducting wire 8 as shown in FIG. 3 . A process of “applying a voltage of 1.5 V between the oxidation electrode 1 and the reduction electrode 7 for 1 hour and then waiting for 1 hour without applying a voltage” is repeated, and the reaction is caused to proceed until the net voltage application time reaches 3 hours. Further, when the application of the voltage is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the voltage is applied again, the aqueous solution 11 is poured into the intermediate tank 4.

Concentration analysis of gas products in each reaction tank was performed using a gas chromatograph at an arbitrary time while the voltage was applied. In particular, a liquid product in the reduction tank 3 was subjected to the concentration analysis by a liquid chromatograph. As a result, it was confirmed that oxygen was generated in the oxidation tank 2. It was confirmed that hydrogen, carbon monoxide, formic acid, formaldehyde, methane, ethylene, methanol and ethanol were produced in the reduction tank 3.

Example 6

Example 6 also uses the same carbon dioxide gas phase reduction device as in Example 5 shown in FIG. 7 . In Example 6, a profile-set voltage was applied to the conducting wire 8 as shown in FIG. 4 . A process of “applying a voltage of 1.5 V between the oxidation electrode 1 and the reduction electrode 7 for 1 hour and then waiting for 3 hours without applying a voltage” is repeated, and the reaction is caused to proceed until the net voltage application time reaches 3 hours. Further, when the application of the voltage is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the voltage is applied again, the aqueous solution 11 is poured into the intermediate tank 4. Other points are the same as in Example 5.

Example 7

Example 1 also uses the same carbon dioxide gas phase reduction device as in Example 5 shown in FIG. 7 . In Example 7, a profile-set voltage was applied to the conducting wire 8 as shown in FIG. 5 . A process of “applying a voltage of 1.5 V between the oxidation electrode 1 and the reduction electrode 7 for 1 hour and then waiting for 5 hours without applying a voltage” is repeated, and the reaction is caused to proceed until the net voltage application time reaches 3 hours. Further, when the application of the voltage is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the voltage is applied again, the aqueous solution 11 is poured into the intermediate tank 4. Other points are the same as in Example 5.

Example 8

Example 8 also uses the same carbon dioxide gas phase reduction device as in Example 5 shown in FIG. 7 . In Example 8, a profile-set voltage was applied to the conducting wire 8 as shown in FIG. 6 . A process of “applying a voltage of 1.5 V between the oxidation electrode 1 and the reduction electrode 7 for 1 hour and then waiting for 10 hours without applying a voltage” is repeated, and the reaction is caused to proceed until the net voltage application time reaches 3 hours. Further, when the application of the voltage is stopped, the aqueous solution 11 is discharged from the intermediate tank 4, and when the voltage is applied again, the aqueous solution 11 is poured into the intermediate tank 4. Other points are the same as in Example 5.

Comparative Example 1

In Comparative example 1, the carbon dioxide gas phase reduction device shown in FIG. 1 is used as in Example 1. As compared with Example 1, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. This configuration is the same as the conventional carbon dioxide gas phase reduction device shown in FIG. 8 . Other points are the same as in Example 1.

Comparative Example 2

Also in Comparative example 2, the carbon dioxide gas phase reduction device shown in FIG. 1 is used. As compared with Example 2, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 2.

Comparative Example 3

Also in Comparative example 3, the carbon dioxide gas phase reduction device shown in FIG. 1 is used. As compared with Example 3, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 3.

Comparative Example 4

Also in Comparative example 4, the carbon dioxide gas phase reduction device shown in FIG. 1 is used. As compared with Example 4, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 4.

Comparative Example 5

In Comparative example 5, the carbon dioxide gas phase reduction device shown in FIG. 7 is used as in Example 5. As compared with Example 5, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 5.

Comparative Example 6

Also in Comparative example 6, the carbon dioxide gas phase reduction device shown in FIG. 7 is used. As compared with Example 6, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 6.

Comparative Example 7

Also in Comparative example 7, the carbon dioxide gas phase reduction device shown in FIG. 7 is used. As compared with Example 7, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 7.

Comparative Example 8

Also in the Comparative example 8, the carbon dioxide gas phase reduction device shown in FIG. 7 is used. As compared with Example 8, the intermediate tank 4 is always filled with the aqueous solution 11 regardless of the light irradiation time and the light stop time. Other points are the same as in Example 8.

Effects

Table 2 shows a current retention rate of the carbon dioxide reduction when the net light irradiation time reaches 3 hours for Examples 1 to 8 and Comparative examples 1 to 8.

TABLE 2 Current retention rate of carbon dioxide reduction reaction (%) Example 1 78 Example 2 76 Example 3 74 Example 4 71 Example 5 78 Example 6 77 Example 7 74 Example 8 73 Comparative example 1 62 Comparative example 2 51 Comparative example 3 34 Comparative example 4 15 Comparative example 5 64 Comparative example 6 55 Comparative example 7 38 Comparative example 8 16

The current retention rate of the carbon dioxide reduction is calculated using Equation (1). It is considered that the larger the value of the current retention rate (%) of the carbon dioxide reduction, the longer the lifespan of the carbon dioxide reduction reaction.

Current retention rate of carbon dioxide reduction (%)=(current value of carbon dioxide reduction 3 hours after light irradiation or voltage application)/(current value of carbon dioxide reduction 10 minutes after light irradiation or voltage application)  (1)

When a concentration of a reduction reaction product is defined as A [ppm], a flow rate of a carrier gas is defined as B [L/sec], the number of electrons required for a reduction reaction is defined as Z [mol], a Faraday constant is defined as F [C/mol], and a molar body of a gas is V_(m) [L/mol], the “current value of carbon dioxide reduction”, which is the right-hand variable of Equation (1), was calculated using Equation (2).

Current Value of Carbon Dioxide Reduction

[A]=(A×B×Z×F×10⁻⁶)/V _(m)  (2)

When Examples 1 to 4 are compared with Comparative examples 1 to 4 in each case of the waiting time of 1 hour, 3 hours, 5 hours, and 10 hours at the time of light irradiation, it can be understood that the current retention rate of the carbon dioxide reduction in each example was improved as compared with each comparative example. Further, when Examples 5 to 8 are compared with Comparative examples 5 to 8 in each case of the waiting time of 1 hour, 3 hours, 5 hours, and 10 hours at the time of the voltage application, it can be understood that the current retention rate of the carbon dioxide reduction in each example was improved as compared with each comparative example as in the case of light irradiation.

From this result, it can be seen that, by filling the intermediate tank 4 with the aqueous solution 11 only for a reaction progress time, which is the light irradiation time or the voltage application time, the lifespan of the carbon dioxide reduction reaction was improved. It is considered that this is because deterioration of the reduction electrode 7 could be inhibited by preventing the aqueous solution 11 from coming into contact with the reduction electrode 7 via the ion exchange membrane 6 during a reaction stop time, which is a non-light irradiation time or a non-voltage application time, in Examples 1 to 8.

As described above, according to Examples 1 to 8, in the carbon dioxide gas phase reduction device that directly reduces gas phase carbon dioxide on the gas reduction sheet 100 in which the reduction electrode 7 is directly formed on the ion exchange membrane 6, the intermediate tank 4 capable of pouring and discharging the aqueous solution 11, which is the electrolytic solution, is disposed between the oxidation tank 2 and the reduction tank 3, and the reduction electrode 7 and the aqueous solution 11 are brought into contact with each other only when the oxidation electrode is irradiated with light or when a voltage is applied between the oxidation electrode and the reduction electrode, and thus recovery of oxygen generated by the oxidation electrode 1 is not hindered, deterioration of the reduction electrode 7 can be inhibited, and the lifespan of the carbon dioxide reduction reaction on the reduction electrode 7 can be improved.

Others

The present invention is not limited to the above Examples 1 to 8, and many modifications can be made within the scope of the gist thereof. The nitride semiconductors shown as the oxidation electrodes 1 of Examples 1 to 4 may have different laminated structures, or may have different compositions such as a composition containing indium and aluminum. Further, for the oxidation electrodes 1 of Examples 1 to 4, instead of the nitride semiconductors, compounds exhibiting photoactivity such as titanium oxide and amorphous silicon may be used. The oxidation electrode 1 of Examples 5 to 8 may be a metal such as gold, silver, copper, indium, or nickel instead of platinum. Instead of copper, the reduction electrode 7 may be gold, platinum, silver, palladium, gallium, indium, nickel, tin, cadmium, an alloy thereof, or a mixture of these metals or metal oxides and carbon. As the aqueous solutions 10 and 11, instead of the potassium hydroxide aqueous solution, a sodium hydroxide aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution may be used. The ion exchange membrane 6 is, for example, a cation exchange membrane called Nafion, which is a perfluorocarbon material composed of a hydrophobic Teflon skeleton composed of carbon-fluorine and a perfluoro side chain having a sulfonic acid group. The method for producing the gas reduction sheet may be an electroplating method, a physical vapor deposition method, or a chemical vapor deposition method, in addition to the electroless plating method.

REFERENCE SIGNS LIST

-   1 Oxidation electrode -   2 Oxidation tank -   3 Reduction tank -   4 Intermediate tank -   5 Ion exchange membrane -   6 Ion exchange membrane -   7 Reduction electrode -   8 Conducting wire -   9 Light source -   10 Aqueous solution -   11 Aqueous solution -   12 Tube -   13 Gas input port -   14 Aqueous solution input port -   15 Aqueous solution output port -   16 Power source -   100 Gas reduction sheet 

1. A carbon dioxide gas phase reduction device comprising: an oxidation tank including an oxidation electrode; a reduction tank to which carbon dioxide is supplied; an intermediate tank that is disposed between the oxidation tank and the reduction tank and is capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution; an ion exchange membrane disposed between the oxidation tank and the intermediate tank; a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank; and a conducting wire connecting the oxidation electrode to the reduction electrode.
 2. The carbon dioxide gas phase reduction device according to claim 1, further comprising a light source irradiating the oxidation electrode with light.
 3. The carbon dioxide gas phase reduction device according to claim 1, further comprising a power source connected to the conducting wire.
 4. The carbon dioxide gas phase reduction device according to claim 1, wherein the oxidation electrode is an n-type semiconductor.
 5. A carbon dioxide gas phase reduction method performed by a carbon dioxide gas phase reduction device, the carbon dioxide gas phase reduction device comprising: an oxidation tank including an oxidation electrode: a reduction tank to which carbon dioxide is supplied; an intermediate tank that is disposed between the oxidation tank and the reduction tank and is capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution; an ion exchange membrane disposed between the oxidation tank and the intermediate tank; a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank; and a conducting wire connecting the oxidation electrode to the reduction electrode, wherein the method includes performing: a first step of pouring the electrolytic solution into the oxidation tank and supplying carbon dioxide to the reduction tank, and a second step of pouring the electrolytic solution into the intermediate tank only when the oxidation electrode is irradiated with light.
 6. A carbon dioxide gas phase reduction method performed by a carbon dioxide gas phase reduction device, the carbon dioxide gas phase reduction device comprising: an oxidation tank including an oxidation electrode; a reduction tank to which carbon dioxide is supplied; an intermediate tank that is disposed between the oxidation tank and the reduction tank and is capable of having an electrolytic solution poured thereinto and discharging the electrolytic solution; an ion exchange membrane disposed between the oxidation tank and the intermediate tank; a gas reduction sheet in which an ion exchange membrane and a reduction electrode are laminated and which is disposed between the reduction tank and the intermediate tank with the ion exchange membrane facing the intermediate tank and the reduction electrode facing the reduction tank; and a conducting wire connecting the oxidation electrode to the reduction electrode, wherein the method includes performing: a first step of pouring the electrolytic solution into the oxidation tank and supplying carbon dioxide to the reduction tank, and a second step of pouring the electrolytic solution into the intermediate tank only when a voltage is applied between the oxidation electrode and the reduction electrode.
 7. The carbon dioxide gas phase reduction device according to claim 2, wherein the oxidation electrode is an n-type semiconductor.
 8. The carbon dioxide gas phase reduction device according to claim 3, wherein the oxidation electrode is an n-type semiconductor. 